Austenitic alloy steel



AUSTENITIC ALLOY STEEL Michael Korchynsky and Walter Crafts, Niagara Falls,

N.Y., assignors to Union Carbide Corporation, a cor-- poration of New York No Drawing. Filed Dec. 12, 1958, Ser. 'No. 77 ,372

'7 Claims. (Cl. 75-428) This invention relates to an austenitc'steel alloy pos-, sessing a unique combination of mechanical. properties Current developments in the field of high-temperature materials have been guided by two major considerations.

One has been the constantly rising temperaturesand stresses that machines and equipment are subjected to and- States Patent O the other is, primarily, a consideration of the economics;

vwhereby they are fabricated are limited only to casting since the very structural arrangement of the constituents of the alloy which imparts these high-temperature prop- :-erties renders them difiicult to work. Other austenitic steel alloys have been developed which owe their superior high-temperature properties to a careful working operation, frequently at relatively low temperatures, usually difficult and expensive to accomplish. Such alloys i are not suitable for casting. 1

In contrast to the conventional austenitic steels, the so-called super alloys, which contain relatively large quantities of cobalt or nickel in addition to chromiumand one or more of the elements molybdenum, tungsten, tantalum, and titanium, have the required strength to recommend them for use in the fabrication of machine parts, e.g., blades and other parts of gas turbines, which For these reasons, it may be seen that there is a need foran austenitic steel alloy (a) possessing good worka wrought condition; (b) having a betterability than conventional austenitic steel alloys to withstand severe mechanical stress at higher temperatures; (0) containing less strategic materials than super alloys; and (d) possessing good welding characteristics.

Accordingly, it is an object of this invention to provide an austenitic steel alloy possessing the unique combination of properties set forth above.

- -Other aims and advantages of the present invention will be apparent from the following description and ap- -pended claims.

In accordance with the invention, an austenitic steel alloy is produced that is suitable for use in a wrought and cast condition. This alloy possesses excellent stress resistance properties in extended service at temperatures on the order of 1400 F.

The broad and the preferred compositional ranges from 2,955,034 Patented Oct. 4, 1960 TABLE I Broad Preferred Chromium 12 to 15 to 20. Mmwane e 10 to 13 to 17. Nickel 4 to 18; 8 to 12.. Molybdenum"- 2 to 3 to 5. Tungsten 1 to 1.5 to 2 5 Nitro en 0.1- to 0.6.- 0.2 to 0 35 Carbon up to 0.6.. 0.3 to 0 5 Iron Balance... Balance 'The composition of the alloy and the relationship between the constituent elements of the alloy may best be hexpresseid mathematically by the following equation:

" where and x =2 0(percent C+percent N) +percent Ni /3 (percent Mil-12) n In the equation base. value up to and including 1 and has a value of at least 1 or more. In determining the tion.

, ability, i.e., suitablefor use not only in a cast but also in 'l superior rupture strength properties of the subject alloy extremely effective austenitic stabilizer. itwas intended to conserve the strategic material and also reduce the cost of the finished article made therefrom, a

values of y or x to be used in the equation, the weight percent of the elements are substituted in Formulae A values selected must satisfy this equation to result in the production of an alloy within the scope of this invention.

The subject austenitc steel alloy is capable of withstanding severe mechanical stress at temperatures on the order ot 1400 F. At this temperature, the subject austenitic steel alloy has and l000-hour rupture strengthspf at least about 25,000 and 20,000 p.s.i., respectively. In

addition, to obtain such an alloy only about 10 percent of the element nickel is necessary in the alloy composi- The following Table II will further illustrate the as compared with prior austenitic steels and with, super alloys.

Nickel, one of the alloy constituents, is a Well-known However, since manganese-nitrogen combination was used as a substitute for part of the nickel. The alloy base from which the desired properties were obtained was a 15 percent-chromium-l5 percent maganese-IO percent nickel-balance iron combination.

Full use was made of nitrogen as an effective alloy addition, and depending on the manganese plus chromium content, amounts up to 0.6 percent nitrogen were found to be suitable for air-melted alloys and did not resultin gas evolution during solidification. Other standard melting procedures may be employed; however, based on economic considerations, air-melting is preferred.

With a nitrogen content of about 0.3 percent, -a re- .markable increase in the rupture strength was produced +1.7(percent W) (A) detrimental.

TABLE II RUPTURE STRENGTH AT 1400 F. (P.S.I.)

100 Hours 1,000Hurs Percent Percent Nl-i-Co Fe 20, 000. 10, 500 11 0s. 4 20, 000 14, 000 25 4s. 9 29,000 17,000 28 50.0 20, 000 20,000 40 25. 5 27,000 20, 000 44 24. s as, 000 25,000 00 5. 0 max I COMPOSITION PERCENT o N Mn Or Ni 00 Mo W Cb'l'Ia T1 Cu v Fe by increasing the carbon content from 0.2 percent to 0.55 percent. time properties and stress-rupture properties increased with increasing increments of carbon. The efiect of the carbon content, on the short-time tensile and stressrupture properties of an iron base alloy containing be tween about 14.9 and 15.6 percent chromium, 13.8 and 17.6 percent manganese, 10.0 and 10.6 percent nickel, and 0.28 and 0.36 percent-nitrogen at 1400 F., is illustrated in Table III below. The yield strength values presented in this table are based on an offset of .2 percent and the elongation values are based on a gauge length of one inch.

TABLE III Efiect of carbon content on the short-time tensile and preciably stronger than the tungsten-free modifications.

It has been found thatboth the shortstress-rupture properties of the CrMn-Ni-NaFe-base alloy at 1400 F.

.To the above chromium-manganese-nickel-carbonnitrogen-iron-base alloy, elements of the group consisting of molybdenum, tungsten, tantalum, columbium, and vanadium were added, separately and in combination, for the purpose of impartinga strengthening efiect to the alloy. Of these, a combination of molybdenum plus tungsten proved to be the most eifective addition in in creasing rupture strength.

The proportions of these two ingredients proved to be 1 critical. The 100- and 1000-hour rupture strength of the steels modified with molybdenum only increased with the increase in the molybdenum content up to 4 percent. Larger amounts of this element, i.e., 6 percent, 'proved The effect of molybdenum on the behavior of steels containing 2 percent tungsten follows, in general, a similar pattern. At 2 percent and 4 percent molybdenum levels, the nmgstenbea'ring..steelsuwereap- The drop in strength at the 6 percent molybdenum level was more pronounced in the presence of tungsten. A

further increase in the tungsten addition to 4 percent lowered the rupture strength andshiftedthe optimum molybdenum content to levels below 4. percent.

Rupture tests were conducted on the alloy at 1400 F. In these tests, standard one-quarter inch tensile specimens were used at stresses ranging from 35,000 to 15,000 pounds per square inch to produce rupture times from several hours to over 1000 hours. Stress-rupture tests were also performed, under'the same conditions on investrnent-cast samples of substantially the same com.- position. The results of these tests are set forth in Table IV below. The yield strength values presented in this table are based on an oflset of .2 percent and the elongation values are based on a gauge length of one inch.

In addition to the unique combination of mechanical properties of the alloy of this invention, it has also been .found'that the alloy exhibits excellent welding characteristics. 1115 response of the alloy to'welding was assessed by means of an annular weld'cr'acking test. The test consists of depositing a -weld bead in-a circular grooye,; machined" in-a heavy plate. The drastic c001- .ingvconditions and the constraint due to thestifiness I of the plate create stress inducive to weld cracking. The

rate of susceptibility of steel to weld cracking is based on the number of cracks formed during the test, as revealed by visual examination.

The test specimens were machined from a 1-inch thick plate, hot-rolled between 1030 to 1210 C. from a 3-inch by 3-inch air-melted ingot of the following composition,

= given in weight percentr-carbon032, nitrogen 0.37,

chromium 15.15, manganese 13.72, nickel 9.77, molybdenum 4.33, and tungsten 2.63. The welding rod was fabricated from the same heat.

An automatic submerged are electric welding process was used to deposit the annular bead at welding speeds of 22.2 and 34.8 inches per minute.

The resultant weld was free of cracks when deposited at the lower speed of 22.2 inches per minute. At the higher welding speed, two cracks were formed, at the start and at the end of the head. The results of these two tests were rated excellent and fair, respectively. It should be noted, however, that the higher speed is well in excess of that being used in structural welding of this type of material.

In making the improved subject alloy, a 14 to 16 pound heat was air melted without slag in an induction furnace using a magnesia crucible. The charge consisted of the following virgin materials: technical grade iron, electrolytic nickel, low-nitrogen and high-nitrogen grades of chromium and manganese, nickel-molybdenum master alloy, tungsten powder, and silicon metal. Care was taken not to superheat the melt since an increase in temperature tended to decrease the amount of nitrogen dissolved in the molten steel alloyed with chromium and manganese. The nitrogen addition was made as nitrogenbearing chromium and manganese at a temperature of about 1550 C., just prior to the final deoxidation with about 0.4 percent silicon.

The heat was cast into 3-inch square split-type cast iron molds. The ingots were forged into one and onequarter inch square bars, in the temperature range from 1050 to 1250 C. and subsequently rolled to five eighths inch diameter bars in the same temperature range.

What is claimed is:

1. An austenitic steel alloy suitable for use in a wrought and cast condition, said alloy possessing superior stress resistance properties in extended service at temperatures on the order of 1400 F., said alloy consisting essentially of, by weight percent, 12 to 25 chromium, to f 20 manganese, 4 to 18 nickel, 2 to 6 molybdenum, 1 to 4 tungsten, 0.1 to 0.6 nitrogen, up to 0.6 carbon, and the balance iron with incidental impurities, wherein the relationship of the constituent elements in said alloy must satisfy the following equation:

where y: 1 .6 (percent Cr- 12.5)

+3.5 (percent Mo) 1.7 (percent W) and x=20(percent C+percent N) +percent Ni+ /a (percent Mn-l2) 16 sls a: a: 1 where y=1.6(percent Cr--12.5) i V +3.5 (percent Mo)+1.7(percent W) and x=20(percentC I- ercent'N) +percentrNi+% (percent Mn-flZ) 3. An austenitic steel alloy suitable for use in the wrought and cast condition and characterized by superior stress resistance properties, in extended service at temperatures on the order of 1400 F., said alloy having a rupture-strength, after 1000 hours under stress at 1400" F., of at least about 20,000 p.s.i., and after hours under stress at 1400 F., of at least about 26,000 p.s.i., said alloy consisting essentially of, by weight percent, 15 to 20 chromium, 13 to 17 manganese, 8 to 12 nickel, 3 to 5 molybdenum, 1.5 to 2.5 tungsten, 0.2 to 0.35 nitrogen, 0.3 to 0.5 carbon, and the balance iron with incidental impurities, wherein the relationship of the constituent elements in said alloy must satisfy the following equation:

where 1.6 (percent C-r- 12.5

+3.5 (percent Mo) +1.7(percent W) and x=20(percent C+percent N) +percent Ni+ /a (percent Mn-12) .2 to 6 molybdenum,1 to 4 tungsten, 0.1 to 0.6 nitrogen,

up'to 0.6 carbon, and the balance iron with incidental impurities, wherein the relationship of the constituent elements in said alloy must satisfy the following equation:

where y: 1.6 (percent Cr--12.5)

+3.5 (percent Mo) 1.7 (percent W) and x=20(percent C+percent N) +percent Ni+ /a (percent Mn-12) 6. A structure comprising metal elements welded together, the weld being possessed of superior crack resistance and consisting essentially of, by weight percent, 15 to 20 chromium, 13 to 17 manganese, 8 to 12 nickel, 3 to 5 molybdenum, 1.5 to 2.5 tungsten, 0.2 to 0.35 nitrogen, 0.3 to 0.5 carbon, and the balance iron with incidental impurities, wherein the relationship of the constituent elements in said alloy must satisfy the following equation:

sls 37 7. A structure oompn'sing metal'elements' welded t0- References Cited the file (if-this patent UNITED STATES PATENTS 2,009,974 'PaysonaJ. July 30, 1934 

1. AN AUSTENTIC STEEL ALLOY SUITABLE FOR USE IN A WROUGHT AND CAST CONDITION, SAID ALLOY POSSESSING SUPERIOR STRESS RESISTANCE PROPERTIES IN EXTENDED SERVICE AT TEMPERATURES ON THE ORDER OF 1400*F., SAID ALLOY CONSISTING ESSENTIALLY OF, BY WEIGHT PERCENT, 12 TO 25 CHROMIUM, 10 TO 20 MANGANESE, 4 TO 18 NICKEL, 2 TO 6 MOLYBDENUM, 1 TO 4 TUNGSTEN, 0.1 TO 0.6 NITROGEN, UP TO 0.6 CARBON, AND THE BALANCE IRON WITH INCIDENTAL IMPURITIES, WHEREIN THE RELATIONSHIP OF THE CONSTITUTENT ELEMENTS IN SAID ALLOY MUST SATISFY THE FOLLOWING EQUATION: 